Method of treating sheets of ligno cellulose



G. GHOLM Nov. 15, 196.0

METHOD OF TREATING SHEETS 0F LIGNO CELLULOSE Filed Jan. 10, 1956 JzmUnited States Patent METHOD OF TREATING SHEETS OF LIGNO CELLULOSE Gustavjholm, Bromma, Sweden, assignor to Aktieholaget Svenska Flaktfabriken,Stockholm, Sweden Filed Jan. 10, 1956, Ser. No. 562,534 Claims priority,application Sweden Jan. 10, 1955 1 Claim. (Cl. 162-206) The presentinvention relates to a new and improved method for drying andpre-heating sheets of ligno-cellulose (wall-boards), to be fed into aheat-press for a treatment in accordance with the so called dry-pressureprocess. When manufacturing hard wall-boards in accordance with thismode boards are usually first manufactured as porous boards. The boardscan be fed to the heatpress either directly from a dryer or from a storeof earlier dried boards. The used press is substantially designed in thesame manner as the presses, which are used for the manufacture of hardboard in accordance with the so called wet method. The press is thusprovided with a plurality of horizontal, superposed heating plates. Acommon number of heating plates is 21, and thus 20 sheets of hard boardscan be manufactured simultaneously in the press. In order to give thesheets as good strength as possible and in order to diminish theirhygroscopic properties the temperature of the sheets must be raisedconsiderably above 100 C. This heat treatment is usually performed inthe press. As there is no evaporation of moisture worth mentioning inthe press in the dry-pressure process, the main part of the heatsupplied in the press is used merely for heating the sheets. Thisheating will take a certain time. This time is rather long, owing to thelow heat conductivity of dry ligno-cellulose. In order to reduce thepress time as much as possible the boards have lately been pre-heated bymeans of alternating current of high frequence. This pre-heating isperformed in such a manner, that a number of sheets are piled upon eachother. Such a package of sheets is placed between two electrodes sodesigned that their plane surface substantially corresponds to the sizeof the sheets. After the package of sheets have been heated by thehigh-frequence electric current, the package is discharged from theheating device and the sheets are fed one by one into a rack located infront of the press. In this feeding rack the sheets are resting overeach other to a number corresponding to that number of sheets, which canbe treated at one and the same time in the press and all the sheets arethen fed simultaneously.

The main draw-back of this known method for preheating is that thenecessary apparatus is rather expensive.. The efficiency of the electricgenerating means is furthermore low. To this comes the disadvantage--which seen from the view of quality is very seriousthat the sheets havedifferent temperatures, when they are fed into the press, even if thesheets have been given one and the same temperature in the heatingdevice. The reason for this is that those sheets, which are firstlyseparated from the preheated package, have had time to cool down beforethe feeding rack in front of the press has been entirely filled and thesheets have been fed into the press. The main object of the invention isto eliminate these drawbacks and the invented method is characterized inthat the treatment is performed in two separate steps heat treatment andpressing-wherein the sheets are heatlCC treated in a chamber arranged infront of the press, in which chamber the sheets are treated in batchesof supported, superposed sheets of the same number as can be fedsimultaneously into the press, whereafter the sheetshaving reached thepredetermined temperature-in batches are transferred directly to thepress for the final treatment.

The drawing illustrates an apparatus for performing the method inaccordance with the present invention.

An apparatus for the performance of the method is illustrated in thedrawing and comprises a heat press 10 of multi-deck type and a heattreatment chamber 14 arranged in front of said press 10. The press isprovided with a plurality of horizontal superposed heating plates 11which are pressed against the opposite sides of the boards between theplates 11 by suitable pressure means 12. The apparatus is mainlycharacterized by superposed conveyors 15 in the heat treatment chamber14 of the same number as the decks of the press 10, which conveyors aredriven by one and the same driving device 16 for the transference of thesheets in batches directly into the press, and further by heatingelements 17 in said chamber for heating the sheets by radiation orheating elements 18 together with means 19 for circulating a heatingmedium for heating the sheets by convection, or by both of these heatingmeans in combination. The pressing plates 11 of the press may be heatedor the pressing plates may be unheated and the press in full enclosed ina heat chamber 14.

By carrying out the method according to the invention for pre-heatingthe sheets the treatment time in the press will be very short. Thecapacity of the press will, therefore, be greater and it can benecessary to build the heating chamber to have room for a number ofbatches of sheets in the length direction of the chamber in order to getsufficient time for the heating. If the heating chamher is designed inthis manner the sheets are to be transported through the chamberintermittently, each time a distance corresponding to the length of asheet. If the heating time for instance is four times as long as thetime for pressing, it will be necessary that the heating chamber hasroom for four batches of sheets. If the sheets are to be heated to sucha high temperature, that there is a risk for dis-colouring by a greaterheat supply at those points, where the sheets are resting on thesupporting means, it may be convenient to slowly move the sheets forandbackwards on the conveyors between the above mentioned intermittentmovements.

The scope of the invention will be evident from the succeding claim.

What I claim is:

A method for making dense hardboard sheets of ligno cellulose or similarmaterials by the dry-pressure process in a multi-deck press having anumber of decks comprising the steps of dividing the process into twoseparate stages by disposing a like number of previously-driedrelatively thick sheets in superimposed spaced-apart relation in astack, heat-treating said dry sheets in the stack in one stage uniformlyto a predetermined temperature suflicient to enable reduction of saidthick sheets to their final thickness and density, thereaftersimultaneously advancing said stack of heat-treated sheets at saidpredetermined temperature directly without substantial loss oftemperature into the multi-deck press having pressing plates on oppositesides of each sheet, and reducing said sheets to their final thicknessand density in a separate stage by subjecting the sheets to pressure bysaid plates and heat at said predetermined temperature in said press.

(References on following page) UNITED STATES PATENTS 1 Hope -L Sept. 21,1920 Loetscfier J ne 14, 19 32 5 Mason Jan. 17, 1933 Ellis Mar. 7, 1933Becher Nov. 20, 1934 4 Mason Dec. 13, 1938 Linzell et a1 Apr. 23, 1940Gill Sept. 17, 1940 Beernink Feb. 3, 1948 Nofziger Sept. 20, 1955Klarmann Apr. 22, 1958 McCormack June 30, 1959

